Wire-grid inorganic polarizing plates provide polarization splitting performance over a wide wavelength range as compared to polarizing plates using organic substances, and have excellent heat resistance as they are composed of only inorganic materials. Wire-grid inorganic polarizing plates are thus used in liquid crystal display devices requiring high reliability and durability, such as transmissive liquid crystal display projectors. In a liquid crystal display device, the light reflected from a polarizing plate causes deterioration in contrast upon return to the liquid crystal panel. Thus, an absorptive polarizing plate is used to absorb polarization components not transmitted through the polarizing plate.
A known absorptive inorganic polarizing plate has a layered structure that includes: a wire grid composed of a reflective metallic film; a dielectric film and an absorptive metallic film stacked on the wire grid; and a dielectric protection film covering these elements. As increasingly more applications are developed, liquid crystal display devices are demanding higher performance and higher intensity. Accordingly, polarizing plates used in liquid crystal display devices are also required to offer high performance such as high extinction ratio, high transmittance, or the like, and high heat resistance. Inorganic polarizing plates have better heat resistance than polarizing plates composed of organic materials. However, when subjected to high heat load for a long time, a reflective metallic film and a absorptive metallic film in the inorganic polarizing plate may change in optical properties due to thermal oxidation. Therefore, techniques have been proposed to improve the heat resistance of inorganic polarizing plates (see, for example, JP201298469A (PTL 1) and JPH1073722A (PTL 2)).
In PTL 1, for example, a reflective metal layer and a absorptive metallic layer are respectively covered by a dielectric layer as the oxide of the metal constituting the reflective metal layer and a dielectric layer made of the oxide of the material constituting the absorptive metallic layer. In this way, the invention described in PTL 1 makes it difficult for the oxide films on the surfaces of the reflective metal layer and the absorptive metallic layer to grow even if the temperature of the polarizing element rises during use, and optical property fluctuations can be suppressed. In PTL 1, for surface oxidation, an oxide film is formed at low temperature by ultraviolet light irradiation in an oxide-containing atmosphere (ozone gas atmosphere).
In PTL 2, a grid made of metal (Al) is formed on a substrate and subjected to heat treatment at 500° C. in an electric furnace in a dry oxide atmosphere to oxidize the surface, thereby forming a protection film. In this way, the invention described in PTL 2 suppresses the growth of an oxide film in the metal lattice if the temperature of the polarizing element rises during use, thereby suppressing optical property fluctuations.